The present disclosure relates to a non-aqueous electrolyte secondary battery and in more detail, to a non-aqueous electrolyte secondary battery which is excellent in heat resistance, liquid-holding properties and cycle characteristic.
In recent years, a number of portable electronic devices, for example, camcorders (video tape recorders), cellular phones and portable computers, each achieving a reduction in size and weight, have appeared. Following this, development of batteries, in particular, secondary batteries as a portable power source for such electronic devices has been actively conducted. Above all, lithium ion secondary batteries have attracted much attention as a device capable of realizing a high energy density.
Also, recently, more reduction in size and weight and thinning of a battery is being advanced by using a laminated film or the like in place of a metal-made battery can made of aluminum or iron as a battery exterior material.
On the other hand, in order to increase the energy density, it is necessary to charge a more amount of an active material which acts to a charge-discharge reaction. Following this, it is known that it is necessary to use an electrolytic solution in an amount sufficient for moving a lithium ion between a positive electrode and a negative electrode. When the amount of the electrolytic solution is not sufficient, and the electrolytic solution does not completely fulfill the surroundings of the active material, a portion which does not come into contact with the electrolytic solution does not react, and a sufficient battery capacity is not obtainable.
Furthermore, as the repetition of charge and discharge proceeds, the electrolytic solution is consumed between the positive electrode and the negative electrode. Thus, the discharge capacity of the battery is gradually lowered before the positive electrode and negative electrode active materials have reached deterioration, thereby causing a problem of a lowering of the cycle characteristic or the generation of an internal short circuit due to a shortage of the electrolytic solution.
In order to overcome such a problem, it is proposed that in a battery using a metal-made battery can, the cycle characteristic can be improved by controlling the volume of a non-aqueous electrolytic solution relative to the discharge capacity (see, for example, JP-A-2-148576).
On the other hand, in order to meet the foregoing requirement for realizing a high capacity of the battery, thinning of a separator is inevitable. However, the higher the capacity is, the larger the energy amount within the battery is. Therefore, at an abnormal time such as short circuit and overcharge, the possibility of more excessive generation of heat generation than before is large.
For that reason, several kinds of means for securing safety even at an abnormal time are applied to the battery, and one of them is a shutdown function of the separator. The shutdown function is a function in which when the temperature of the battery increases due to some factor, pores of the separator are clogged, and a battery reaction is stopped by inhibiting the movement of an ion, thereby suppressing the excessive heat generation.
Polyethylene has been frequently used for a separator of a lithium ion secondary battery because it is excellent in such a shutdown function. However, there may be the case where polyethylene is exposed to a temperature at which the shutdown function is revealed or higher. In that case, there was involved a problem that the separator causes heat shrinkage, and the exposed positive electrode and negative electrode come into contact with each other to generate an internal short circuit, thereby causing thermorunaway.
On the other hand, polypropylene is exemplified as a material having a high melting point and capable of suppressing the heat shrinkage until a higher temperature. However, inversely, the temperature at which the shutdown function is revealed becomes high.
Then, in order to solve these problems, a separator composed of a laminate of polyethylene and polypropylene is proposed (see, for example, JP-A-10-261395).
In the foregoing problems of the related art, different from the case of batteries using the foregoing metal-made battery can, it is hard to say that a lowering of the cycle characteristic or the generation of an internal short circuit due to a shortage of the electrolytic solution is sufficiently solved in a non-aqueous secondary battery using a laminated film.
That is, when damaged, the laminated film is easily broken as compared with firm metal-made cans, and liquid leakage from a broken portion thereof is easy to occur. Accordingly, there was involved a problem that when the amount of the electrolytic solution is increased for the purpose of increasing the cycle characteristic, the liquid leakage easily occurs.
On the other hand, in order that the revealment of a shutdown function of a separator and the suppression of heat shrinkage properties may be regulated or made compatible with each other, when a laminate of polyethylene and polypropylene is used, the separator itself becomes thick. Thus, there was involved a problem that the energy density of the battery is rather lowered.
In view of the foregoing problems of the related art, it is desirable to provide a non-aqueous electrolyte secondary battery which is excellent in heat resistance, resistance to liquid leakage and cycle characteristic.